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EBM_dude 31st August 2009 01:49 PM

Chipamp input impedance question
This is a completely noob question, but please bare with me as I'm completely clueless at the moment. I'm going to build a LM3886 chipamp. In the datasheet, they use a 10K pot for volume regulation. I'm going to use a 470k pot, because I can get good quility ones for very little money. In the datasheet they also give an example how to calculated this and that but I simply don't get it. Can anyone get me started on that topic, i.e. a few good reads on op amps, impedance and volume pots?

AndrewT 31st August 2009 04:40 PM

look at fig2 page6 of the 3886 datasheet.
It shows all the components listed in the table of external components page8.
ignore that this schematic has a single supply, I presume you will be using a dual polarity supply as detailed in fig1.

The Rin of the attenuator is set by the resistance of the pot.
The Rin of the amplifier is set by Ra(75k)+[Ra91k//Ra100k] ~ 120k
This is about 10times the pot value.
It is usual to make the amplifier Zin (or Rin) about 5times to 20times the source impedance (resistance).
Your 470k pot will have an output impedance of zero to [470k/4], i.e. varies from zero ohms to 115k. The output impedance of the pot is the source impedance (Rs) that the amplifier sees. The minimum Zin of the amplifier should be >= 500k (>= 5*115k). This is a bit higher than modern solid state amplifiers adopt. But it will work.

The parasitic capacitance between your 470k pot and the input (pin10) of the amplifier must be kept very low. That means you must make all the connections very short and use low capacitance techniques/materials to allow your high impedance amplifier to work well.
I would recommend a pot value from 10k to 50k to help avoid the build complexities that a 470k pot will impose on you.

Read Pass, Self and ESP for good information on the basics as well as a more thorough coverage of the principles.

EBM_dude 31st August 2009 05:41 PM

Thanks, that helped me really much! I'll give up on the 470K pots then and look for better alternatives then. I feel that I don't have the necessary skills for such a build yet. rules!

ashok 6th September 2009 03:43 PM

Is that a linear 470 K pot or a log type ?
If it's linear then you can use a low input impedance load to get an approximate log volume pot. This is covered somewhere on this forum and I don't want to repeat it. Search for it. Typically 1/10 th the pot value IIRC .

Component values are not cast in stone. You can play around with them.
What are you using ? A split supply or a single supply ?

EBM_dude 24th October 2009 11:57 AM

Hm, I think I'll stick to the conservative route and use a log pot anyways. I use a split power supply.

Geoffroy 24th October 2009 12:42 PM

I dare add my own question to this thread.... I have built a lm1875 gainclone with BrianGT's PCB's and I a did add a 100K log pot on the input to control the volume.

It works well, however could this have an influence on the quality of the sound ? Basic question for beginner off course but I'd be happy to understand what is the influence of the pot........



AndrewT 26th October 2009 04:52 PM

the high output impedance makes the combination of attenuator and receiver very susceptable to small small changes/additions to the capacitance on the input of the receiver or the connections back to the attenuator.

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